First, that's not a relationship honey.
Second, I'd be devastated if my boyfriend said he didn't mind if I slept with multiple others.
Third, I thought perhaps he didn't care about you or the relationship and was secretly sleeping around himself...until you described his shy, low confidence personality. Now I suspect he probably does care a bit, but is so desperate to keep you he is being an absolute idiot thinking you will leave him to get with other people at uni. Therefore, in order that you don't leave him, he is giving you "permission" to sleep with others to satisfy yourself, and in actual fact he would be pretty devastated, but would suffer in silence because he weirdly doesn't want to lose you.
But then, he is weak and is misunderstanding the concept of an adult relationship by about 10 miles. He sounds quite immature and oddly attached/obsessed with keeping you that he'd do anything - something quite strange and not at all what a relationship is about.
It's already heading that way, but if you sleep with someone else you've ended any relationship that was there. Even if you continue to be "in a relationship" with him, you never will be. Eventually you will realise you don't need him after months of hurting a poor confused immature boy and dump him. Want to save this relationship? Don't do it and tell your boyfriend you want him and only him. Want to destroy it slowly? Cheat on him.
I think you should just end it now.